Laser doppler velocimetry (LDV) in gases at long ranges has been a subject of investigation for nearly 20 years. Early velocity measurements were performed in large wind tunnels at distances of a few meters. Subsequent investigations included the measurement of meteorlogical parameters including wind velocity and turbulence, in part because of their importance to aircraft flight. By the early 1970's, laser doppler velocimeters were operating at ranges of hundreds of meters and, by the early 1980's, measurments were being made at kilometer ranges. These systems were quite large and were characterized by weights of thousands of pounds.
A typical long-range LDV for measuring wind shear includes a source of polarized radiation, which in current technology would be either A CO.sub.2, YAG, or argon laser, for projecting a first coherent beam of light into a beam shaper. The beam shaper expands and collimates the beam after which the beam enters a telescope. The telescope projects the beam in nearly collimated form. A scanning mirror positioned after the telescope aims the beam at a point of interest. The beam strikes airborne particulates at the point of interest which results in a scattered beam. The scattered beam is received either by the telescope or a separate receiver telescope. The scattered beam is then directed to an optical mixer where it is mixed with a separate reference beam of light. Since the scattered beam is Doppler shifted by the relative velocity of the aircraft and the particulates, it has a slightly different frequency. When the scattered beam is mixed with the reference beam, the two are heterodyned producing a beat frequency, i.e., the Doppler frequency. The reference beam can be frequency shifted to adjust the location in the frequency domain of the Doppler signal to improve velocity resolution and simplify the processing of the signal. The optical mixer is coupled to a photodetector which produces an electrical signal proportional to the Doppler frequency, which may then be displayed.
The source of the reference beam is a major problem in this type of system. Originally, the reference beam is mutually coherent, i.e., in substantially phase identity, with the first beam. However, the reference beam becomes depolarized over time which results in significant error. To overcome this problem in some systems, the lasers are made with an extremely long coherence length and, therefore, one simply mixes a portion of the laser light split from the laser with the return wave to attain interference. Alternately, a second laser is used to generate the reference beam provided that it can be properly phase-locked to the laser source. Neither of these two methods has, however, proven practical. The detected heterodyne signal is typically very weak and noisy and requires amplification and filtering. In addition, the laser's relatively large size precludes its use on aircraft.
In U.S. Pat. No. 4,329,664 entitled "Generation of Stable Frequency Radiation at an Optical Frequency" by A. Javan, the accuracy of LDV systems is improved by correcting the error introduced by laser chirping. The system includes a power laser which produces optical radiation at a frequency that can fluctuate over a short interval, a reference laser for generating a modulation signal in the radio frequency range having frequency variations corresponding to the fluctuations of the power laser and a modulator for generating the desired stabilized optical radiation. Also, an optical delay in the form of mirrors is inserted between the power laser and the modulator to lengthen the optical path to correct the delay time introduced by laser chirping. However, there still exists significant error in the measurement of the wind velocity, because, despite the correction of power laser radiation, it still is not mutually coherent with the reference beam used to produce the heterodyned signal. Consequently, the heterodyned signal is very weak and noisy.
Therefore, it is an object of the present invention to produce a wind shear detection system using a laser for remote detection of a wind velocity gradient such as wind shear which eliminates the above-mentioned coherence limitation.
It is another object of the present invention to produce a wind velocity gradient detection system which is small enought to be considered for flight applications.
It is a further object of the present invention to produce a wind velocity gradient detection system which is capable of several different distance measurements.